Exploring What's Missing: What Do Target Absent Trials Reveal about Autism Search Superiority?

Background: Individuals with autism spectrum disorder (ASD) excel at visual search, performing better than their typically developing (TD) peers across the lifespan. This disparity in performance generally becomes greater as target-distractor similarity increases and when the target is absent.

Objectives: First, to assess the roles of enhanced stimulus discrimination and peripheral selection in superior visual search among individuals with ASD, and, second, to examine the search mechanisms underlying the well-documented superiority of individuals with ASD to discern target absence.

Methods: We administered a paradigm in which distractors varied on difficulty of discrimination and selection to 22 school-age children and adolescents with ASD and an age- and non-verbal IQ-matched comparison group of 30 TD children. The stimulus array had a fixed set size of 24 items. The target was a circle and the distractors were “C” shaped figures. Peripheral selection was varied by increasing the line-width of distractors; discrimination between target and distractors varied by increasing the size of the gap in the distractor Cs (Figure 1a). The experiment was divided into four 40-trial blocks.

Results: Groups did not differ in error rate. Children with ASD were faster at visual search than their TD peers, F(1,50) = 3.5, p = .068. However, there were no significant interactions between group and discrimination, selection, or a three-way interaction between group, discrimination, and selection (all p>.05). Rather, consistent with the large majority of ASD visual search, group differences were mainly the effect of faster ASD performance on target absent trials, F(1,50) = 6.7, p = .013. Next, we conducted a series of analyses to examine the search characteristics that confer a unique advantage in absent trials to individuals with ASD. Faster RT to target absent trials was not due to speed-accuracy trade-off, response bias, quitting threshold, or post-error slowing (all p>.05). Instead, we found faster RT to targets appearing on the left side of the array for our TD group. However, this left-side bias was not present in individuals with ASD (Figure 1b-d). Consistent with our RT results, TD individuals, but not individuals with ASD, showed a leftward bias in saccadic endpoint (Figure 1e).  Lastly, ASD symptom severity was positively associated with ASD search superiority (Figure 1f).

Conclusions: Consistent with previous reports, we found accelerated search in ASD, which was driven primarily by faster responses to target absent trials. Behavioral and eye-tracking results suggest that absent advantage may be due to reduced leftward search bias in ASD. Because target proximity is an important factor in determining subsequent target fixation and search termination, non-biased saccades are likely to make targets more accessible, facilitating search in ASD. These findings together with fewer fixations and reduced saccadic error in target present trials lends support to the hypothesis that enhanced perceptual load may contribute to superior search in ASD. Additionally, indices of target absent search and reduced search bias were related ASD symptom severity, indicating that mechanisms underlying absent search advantage may have explanatory significance with regard to the development of ASD social and communicative impairments.